Magnetic sector mass analyzers are a well-known class of mass spectrometer, for high resolution analysis of ions across a relatively wide mass range. Ions are accelerated through a flight tube under the influence of an accelerating voltage U0 where they separate in time of flight. The kinetic energy of ions at the end of the accelerating process, mv2/2, is a result of the energy imparted by the accelerating field, z.U0; hence
      z    ·          U      0        =            m      ·              v        2              2  
As the moving ions enter the magnetic field created by a magnet, charged ions of a particular mass to charge ratio m/z are deflected along a circular path of unique radius rm in a direction perpendicular to the direction of the applied magnetic field. As will be well known, the force due to the magnetic field (z.v.B, where z is the ionic charge, v is the ion velocity, and B is the magnetic field strength) balances the centripetal force mv2/rm.
Rearranging for v and substituting into the equation above yields:
      m    z    =                    B        2            ·              r        m        2                    2      ·              U        0            
In other words, ions of a particular mass to charge ratio will follow a curved path of radius rm for a given magnetic flux density B and when accelerated to a particular potential U0.
In a sector mass analyzer, the relative positions of the ion source, accelerator region, magnet and detector are fixed. Hence ions of a particular species will only arrive at the detector (rather than the walls of the analyzer for example) at a specific B and U0.
For detecting a particular mass, it is necessary to control (ie hold steady) the magnetic field to a high degree of accuracy. Of course, the accelerating potential U0 must also be held constant but this is relatively straightforward; also note from the equation above that the position of ions as they arrive at the detector (related to rm) is proportional to B−1 but is less sensitive to changes in U0 as rm depends on the square root of U0.
Both U0 and B can in principle be varied to scan multiple ion species across the detector. However, it is preferable, for a particular experiment, that U0 be held constant whilst B is changed. This is primarily because, in a magnetic sector mass analyzer, the focal point of the ions changes with U0 and it is desirable that the ion beam remain focused on the detector.
It is known in the art to measure the flux density generated by the magnet by an appropriate sensor, and to use this signal to regulate the magnetic field. See for example U.S. Pat. No. 3,597,679. An appropriate sensor is typically either a Hall effect sensor or a Field probe.
A typical set-up is shown in FIG. 1. The flux density within the magnet is measured using a field probe 10 or the like. The analogue signal output from the field probe 10 is amplified and then connected to a first input of an operational amplifier 20. A set value is generated as a digital signal at a microcontroller 30 and is converted to an analogue signal at DAC 40. The output of the DAC 40 is connected to the second input of the operational amplified 20 so that the (analogue) measured flux density can be compared with the analogue set point.
The output of the operational amplifier 20 is amplified in a power amplifier 50 and the power amplifier 50 output is used to control the current supplied to the magnet in the magnetic sector mass spectrometer. The magnetic flux density measured by the field probe 10 can thus be used to control the set value, and, in this manner, an analogue closed loop feedback control is effected. Moreover, proportional-integral-differential (PID) control of the operational amplifier (and in principle the amplification of the measured flux density) is possible.
Although a step resolution of the magnetic flux density achievable in the arrangement of FIG. 1 is limited by the resolution of the DAC 40, nevertheless the stability of the magnetic flux within one DAC step can be much higher, depending, for example, on the operational amplifier 20 and other part of the electronics.
The primary use of magnetic sector mass spectrometry is for the determination of abundances of known elements in samples (in contrast to, for example, Orbitrap® or TOF Mass Spectrometry, which typically seek to identify unknown elements in samples, or to confirm or refute the presence of a particular substance, or, more usually, a group of substances. This is why maximum stability of the magnetic field is desirable: a highly constant magnetic field at a given U0 will ensure that ions of a particular m/z (and only ions of that specific m/z) are properly focused on the detector, and remain so over time. In consequence, magnetic field stability is directly linked to instrument resolution; for ions of two adjacent masses, the ability to discriminate between them (ie to measure one species but not the other) will be a consequence of magnetic field stability. For an instrument resolution of around 20,000, the standard deviation of the magnetic flux density needs to be stable to within 3 ppm over an hour. For an instrument resolution of 50,000, the standard deviation of the magnetic flux density needs to be stable to within 1 ppm over an hour. Even higher resolutions would be desirable.
In addition to the requirement for a highly stable stationary magnetic field, it is also desirable that the magnet of a sector analyzer be capable of jumping between two stable magnetic flux density values, in order to measure the intensity (quantity) of two different ion species. The jump should be as fast as possible, whilst the magnetic flux density within the magnet following the jump must be stable for the reasons explained above. This is especially demanding, as reorientation of the magnetic domains within the magnet material is governed by a relatively slow time constant.
Remanent magnetization within the magnet core may also present a challenge. In particular, the homogeneity of the magnetic flux density may reduce across the magnet, particularly when jumping from a relatively higher to a relatively lower magnetic field. Furthermore, the magnetic flux density at the position of the magnetic field probe may then differ from the magnetic flux density at the position of the ion beam. The magnet jump will take a considerably longer time when the target magnetic flux density is low.
FIG. 2a illustrates a plot of magnetic flux density, B, against magnetic field strength, H, for 1006 Steel (source: Femm 4.2 (finite element magnetic simulation software, David Meeker). From that figure the non-linearity of B vs H is very clear. FIG. 2b is derived from Paul Oxley et al, Journal of Magnetism and Magnetic Materials 321 (2009) 2107-2114 and illustrates hysteresis effects as the magnetic field jumps. FIG. 2b shows how the H field (Magnetic field strength, measured in A/m, and proportional to magnet current)) necessary to reach a certain flux density value depends on the history (jump size) and, moreover, just how non-linear the behavior is.
The result of this is that a single set of PID parameters is generally inappropriate to cover all possible jump sizes and flux densities. In particular, the PID parameters determined for a jump between a first pair of magnetic field strengths may not be appropriate—or at least, not optimal—for a jump between a second, different pair of magnetic field strengths. Jumping between different magnetic flux densities means that different time constants would have to be considered, e.g. the time constant of the power stage, the time constant of the ordering of the magnetic domains, and the time constant for heating up the magnets.
Furthermore, even once the magnetic field is nominally stationary (ie is not in the process of jumping from one value to another) the PID parameters are fixed for all values of B and U0 . In practice this means that they are at best a compromise, and for certain combinations of target flux densities and jump sizes, may not be very appropriate at all. Even for a given nominal magnetic flux density, optimal PID parameters will depend upon device specific conditions such as the particular location of the field sensor within the magnet gap and so forth.
The electronic circuit of FIG. 1 also itself introduces potential sources of both random and systematic error to the magnetic flux density in the magnet gap. For example, the output of the magnetic field probe always contains some noise. Smoothing is typically done by applying RC elements. It is challenging to optimize the circuitry in order to smooth away the noise, but at the same time not sacrifice time information. Also there are a number of possible sources for drift effects, e.g. due to temperature changes, especially in the field sensor 10, the operational amplifier 20 and the DAC 40 of FIG. 1. Addressing these effects requires elaborate and costly electronic designs, e.g. temperature stabilization and/or calibration.
Typically, the magnetic flux density B in a magnetic sector mass spectrometer will be variable between a few tens of mT and a few hundreds of mT, up to about 1 T. In a currently preferred instrument, for example, the flux density inside the magnet gap is variable between 200 mT and 750 mT. The flux density in the iron core of the magnets is typically higher by a factor of up to 2, depending upon magnet design and geometry. Fast jumps can occur over the full mass range.
A. A. Malafronte, M. N. Martins, Proceedings of 2005 Particle Accelerator Conference, Knoxville, Tenn., page 2833ff describe an arrangement for controlling the magnetic field in a microtron. The output of a Hall sensor is converted to a digital signal using a 16 bit ADC. A digital set point for the control loop is generated and a microcontroller compares the digital set point with the digitized Hall sensor output. A digital control signal is then generated as an output from the microcontroller as a result of the comparison. That digital output is supplied to a 16 bit DAC which provides an output that is amplified in an analogue driver. The output of the driver in turn controls the current to the magnet power supply.
The Malafronte arrangement is aimed at reducing cost. Magnetic field stability and jump speed between different magnetic flux densities are not critical to the considerations in the microtron controller described; flux stability of 30 ppm is noted, which is an order of magnitude or poorer than is needed for a high resolution (20,000 or higher) magnetic sector mass spectrometer.
Against this background, the present invention seeks to provide an improved arrangement and method for control of the magnetic field in a magnetic sector mass analyzer.